1 Answer
1

su (mostly) uses pam for authentication and pam has a module called pam_wheel which checks group membership of the authenticating user. In short, by adding

auth required pam_wheel.so group=becomeroot

to the file /etc/pam.d/su, only users who are members of the group becomeroot may become root using su. Now you make sure only your user EMERG is a member of the group becomeroot. Some distros have/use the group named wheel for that.

groupadd becomeroot #add the group becomeroot to your system
gpasswd -a EMERG becomeroot # add the user EMERG to the group becomeroot

Further reading:
pam (7)
pam_wheel (8)
groupadd (8)
gpasswd (1)
and many distros have explaining comments in /etc/pam.d/su as well